Improvement of hydrogen embrittlement resistance is an important problem with a high-strength steel, such as a steel having a tensile strength not lower than 1800 N/mm2. Hydrogen embrittlement resistance is a property of a steel resistant to time-dependent fracture due to embrittlement of the steel resulting from absorption of hydrogen during use. Various studies have been made to improve hydrogen embrittlement resistance. It is considered, in recent years, that increase in the hydrogen content of a steel with time resulting from the increase of hydrogen penetrated into the steel and capable of moving in the steel, namely, diffusible hydrogen, is one of factors of brittle fracture. Efforts have been made to prevent the embrittlement of a steel by trapping diffusible hydrogen. For example, a technique disclosed in Patent Reference 1 traps hydrogen by precipitating a carbonitride in the layer structure of tempered martensite and ferrite to prevent fatigue fracture.
There are many proposals to use precipitates capable of trapping hydrogen. Technique disclosed in Patent Reference 2 suppresses hydrogen embrittlement by trapping hydrogen with carbonitride of Ti, Nb, Zr, Ta, Hf or Mo.                [Patent Reference 1] JP-A-2003-105485        [Patent Reference 2] JP-A-10-110247        